Ligaments are specialized connective soft tissues which connect different organs or tissues and attach bone to bone. In the latter case, ligaments provide stability to joints by being flexible enough to allow natural movement of the bones yet also are strong and inextensible to prevent resistance to applied forces. Tendons connect muscle to bone and are capable of withstanding tension. In addition, tendons passively modulate forces during locomotion, providing additional stability with no active work. Their elastic properties allow tendons to store and recover energy at high efficiency. In tendons and ligaments, bundles of collagen fibres are embedded in a connecting matrix made of proteoglycans components. These bundles of collagen fibres provide the load carrying elements. In tendons, the collagen fibres are arranged in nearly parallel formation, thus enabling them to withstand high unidirectional loads. In ligaments, the collagen fibres are arranged in a less parallel formation, thereby enabling them to withstand predominant tensile stresses in one direction and smaller stresses in other directions.
Every year, hundreds of thousands of people sprain, tear, or rupture ligaments in particular in the knee, shoulder, and ankle or suffer from injuries to tendons of the upper and lower extremities, in particular in the shoulder, knee, foot, and ankle. One such ligament often affected by these types of injuries is the anterior cruciate ligament (ACL) of the knee. The ACL serves as a primary stabilizer of anterior tibial translation and as a secondary stabilizer of valgus-varus knee angulation, and is often susceptible to rupture or tear resulting from a flexion-rotation-valgus force associated with sports injuries and traffic accidents. Ruptures or tears often result in: severe limitations in mobility; pain and discomfort; and an inability to participate in sports and exercise. More than 200,000 people in the U.S. alone tear or rapture their ACL each year, leading to costs of approximately $3 billion for ACL reconstructive surgery and extensive rehabilitation. It is widely known that the ACL has poor healing capabilities. Total surgical replacement and reconstruction are required when the ACL suffers a significant tear or rupture resulting in joint instability. The most common practice is to reconstruct a torn ACL by substituting the torn ligament with the patient's own tissue, also known as an autograft. Other options for substitute ligaments include donor tissues from another organism, also known as allografts, as well as synthetic grafts.
Surgeons have considered ligament constructs comprising collagen fibres, biodegradable polymers and composites thereof. When it comes to synthetic grafts the graft material is sometimes composed of linear arrangements of natural collagen fibres; however, this arrangement often makes repairing ruptured or lacerated tendons difficult. Also depending upon the nature of the repair, the tensile strength is not optimal. Accordingly, there is a continued need for replacement material, especially for ACLs, which has both the appropriate mechanical strength and also the appropriate physical properties to enable surgical implantation.
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